This invention relates to a method and apparatus for cutting a substrate using laser irradiation. In particular, it relates to such a method and apparatus for use with a substrate bearing integrated circuit components. The invention has particular application in the singulation of integrated circuit components using laser irradiation.
Silicon wafers or integrated circuit (IC) units are typically made up of a number of individual layers. These layers may comprise a printed circuit board (PCB) package upon which are provided some or all of the following; metal circuitry, dielectrics, wafer dies, bonding wires and moulding compound materials. Typically, a number of individual IC units will be formed on one package, which will be marked so as to define the boundaries of the individual IC units. It is therefore necessary to singulate the package so as to separate each individual IC unit.
A known singulation technique is that of mechanical sawing. U.S. Pat. No. 6,140,708 to Lee et al, entitled xe2x80x9cChip Scale Package And Method For Manufacture Thereofxe2x80x9d, discloses a manufacturing process in which the individual units are singulated from an encapsulated package using a diamond saw. This prior technique has many drawbacks. The saw must be manufactured to exacting standards of homogeneity and flatness. Water is also required during the sawing process to clean the sawing debris and to dissipate the heat generated. Another disadvantage is that the high degree of wear requires frequent saw replacement, which leads to high equipment costs. Furthermore, the minimum cut width of the saw imposes limitations on the density of IC unit fabrication. In addition, the mechanical sawing process can lead to cracks, particularly in relation to thinner IC units. In addition, metal substrates have recently gained in popularity due to their low cost. Typically, a substrate will have a copper plate base coated with a layer of nickel. However, metal substrates generate metal debris which can lead to even greater problems. The metal debris tends to stick to the saw blade, leading to damage to both the IC units and the saw blade itself.
Another technique for the singulation of IC units is that of laser singulation. WO 01/10177 (XSIL Technology Limited) discloses a method and apparatus for singulation of IC units using a laser. The laser energy is scanned across the IC package using either rotating or laterally moveable mirrors. This method also has drawbacks. The cutting speeds attained by using this technique are quoted as 4.2 mm/sec and 8.3 mm/sec. Furthermore, the thickness of package suitable for cutting using this technique is limited by the depth of focus of the laser beam. This technique is therefore not suitable for many industrial applications.
There is therefore a requirement for an improved method and apparatus avoiding the above disadvantages. In particular, there is a requirement for a method and apparatus for cutting a substrate using laser irradiation that avoids the problems of diamond-wheel saw dicing (e.g. cost of saw blades, frequent wear, large minimum cut width, cracking, need for water to remove debris and dissipate heat) while providing reasonable cutting speeds and being suitable for use with thicker substrates.
It is an object of the present invention to fulfil the above requirements.
According to the above object, the invention comprises a method of cutting a substrate comprising the steps of:
a) focussing laser energy onto a point on the substrate so as to provide a laser focus point on the substrate;
b) effecting relative lateral movement between the said laser focus point and the said substrate so that the said point follows a first path on the said substrate, a first layer of the said substrate being removed along the said first path so as to reveal a second layer of the said substrate;
c) refocussing the laser energy onto the said second layer, so that the laser focus point is located on the said second layer; and
d) effecting relative lateral movement between the said laser focus point and the said substrate so that the said point again follows substantially the said first path on the said substrate, whereby a second layer of the said substrate is removed along the said first path.
Preferably, steps c) and d) are repeated, a further layer of the substrate being removed with each repetition, until all layers of the substrate have been removed along the said first path.
Advantageously, the laser energy used has a wavelength in the ultra-violet to visible range. Further advantageously, the laser energy used has a wavelength greater than 500 nm, preferably 532 nm
Preferably, a fluid flow is directed towards the cut region. Particularly preferably, the said fluid is air.
According to one embodiment, the substrate is provided with a plurality of Integrated Circuit (IC) units formed thereon, the method being used to singulate the various units.
According to another aspect, the invention provides apparatus for carrying out the above method.